Electromagnetic device



Nov. 29, 1932. F. A. MITCHELL ELECTROMAGNETIC DEVICE Filed Aug. '7, 1928 3 Sheets-Sheet 0 INVENTOR. Frank flllen Hifclzell A TTORNEY Nov. 29, 1932- F. A. MITCHELL 1,889,000

ELECTROMAGNETIC DEVICE Filed Aug. 7, 1928 3 Sheets-Sheet 2 Fig 6 1 INVENTOR. Fran/i JYllen Mitch el B 444/ A TTORNEY NOV. 29, 1932. A, MITCHELL 1,889,000

' ELECTROMAGNETIC DEVICE Filed,Aug. '7, 1928 3 Sheets-Sheet 3 IN VEN TOR.

/ ATTORNEY STATES UNITE ATNT -FECE FRANK ALLEN MITCHELL, OF LONDON, ENGLAND. ASSIGNOR TO COLUMBIA PHONO- GRAPH COMPANY, INC., OF BBIDGEPOBAT, CONNECTICUT, A CORPORATION OF NEW YORK Application filed August 7, 1928, Serial No. 298,062, and in Great Britain August 17, 1927.

This invention relates to electro-magnetic devices particularly for the reproduction or recording of sound, and furthermore has particular reference to that type in which an armature or like moving element is actuated to produce or to record the sound.

The object of the invention is to provide an improved device of the above character which shall give better transmission characteristics than have hit erto been obtained, so that the essential musical frequencies will be reproduced with equal fidelity.

Further objects of the invention will be apparent from the description given hereafter. p

The invention consists in an electrical reproducing device for the interconversion of mechanical and electrical audio frequencies of the type having a polarized armature and magnet characterized in that the armature is polarized by one pole of an electro-magnet having a polarizing field coil, and operates in combination with an audio frequency coil or coils when the armature vibrates relative to the other pole or poles of the electro-magnet.

The invention also consists in an electrical reproducing device of the above type char acterized in that armature is polarized and is pivoted to operate in conjunction with two pole extensions on the magnet in a manner such that the normal magnetic air gaps between the armature and the pole extensions can be adjusted by movement of one or more of the component parts which are specially adapted for that purpose.

The invention also consists in an electromagnetically operated loud-speaker or recording device having a magnet and an audio frequency electrically controlled moving element characterized in that the magnet is polarized by a field coil in which the flow of current is limited by the direct current flow through one or more of the electrodes in the last stage of a thermionic valve amplifier, while the audio frequency component thereof is applied to operate the moving element.

The invention further consists in other details and arrangements hereinafter described with reference to the accompanying drawin gs, in which Figure 1 is a front elevation of an electrical reproducing device in accordance with the invention showing a part of the cover broken away. I

Figure 2 is a sectional elevation on the line 22, Figure 1. v

Figure 3 shows another modification of the invention.

Figure 1 is a sectional elevation on the line 14, Figure 3.

Figure 5 shows the invention applied to a cone type of diaphragm.

Figure 6 is a plan of the above with the diaphragm removed.

Figures 7 and 8 show other application of the invention in which the component parts are differently arranged.

Figures 9 to 12 show several modifications of thermionic valve circuits in accordance with the invention.

Figure 13 illustrates several applications of a horn to electrical reproducing devices in accordance with the invention.

In carrying my invention into effect in one convenient manner I provide a loud-speaking device as shown in Figures 1 and 2 with an electro-magnet 15 of the pot type consisting of a central core 16 and outer cylindrical casing 17 with the field winding 18 therebetween. The inner core or pole 16 may be of material of high permeability and the casing 17 may be of a material of lower permeability such as cast iron, or the whole may be of the same material or otherwise as desired.

An armature 19 of silicon steel, iron, or any other magnetic material is pivotally mounted upon supports 20, 21 which are mounted on the outer cylindrical pole piece 17 so that it is located above but not in contact with the inner magnetic pole 16.

The pivots 22 may be of any suitable kind, but arepreferably mounted upon ball or roller bearings 23 and 24, it being very difficult to obtain a non-resilient bearing in any other manner, although any other. suitable antifriction method of pivoting may be adopted if desired.

By such mounting the armature 19 is maintained at the same magnetic polarity as the outer casing 17, and so of opposite polarity to the central core 16. The central core 16 or other pole piece is provided with two eX- tensions 25 and 26 which project towards the opposite ends 27 and 28 of the pivoted armature 19. Upon these two extensions small coils 29 and 30 are located one on each limb.

By using an electro-magnet, the normal magnetic flux of the system may be adjusted so that it is approximately one-half the magnetic saturation value for the system, at which mid value it is obvious that the maximum change in magnetic flux due to the superimposed force is possible.

When direct current is passed through the field winding 18 the armature 19 will remain balanced above the inner pole, provided that the magnetic air gaps between the pole extensions 25, 26 and the ends 27, 28 of the armature 19 are both equal. If new alternating current is passed through the small coils 29 and 30 which are wound in opposite directions, then one inner pole extension will become north and the other extension will become south. These superimposed magnetic excitations will be added to the steady magneticexcitation of the central core 16 (which may be north for example), and the result will be that one of the pole extensions, 25 for example, will be north north which, assuming the magnetic motive forces to be equal to one another, means that this pole extension 25 has twice its previous netic value. The other pole extension 26 will be north south of equal value so that the magnetic result in this instance will be Zero. One end 27 of the armature is attracted by the strong north pole extension 25 while the other end 28 is released by the pole extension 26. During the next half cycle the position is reversed and the armature 19 rocks in the other direction, and thus follows the wave form of the electrical impulses which are applied by means of the small coils 29 and 30.

To enable the correct distance to be maintained between the extensions and the ends of the armature 19 it is desirable that the extensions 25 and 26 should be adjustable, and they may be made to move together or independently by means of any suitable adjustment. This may be done by screwing the pole extensions 25 and 26 into the inner pole piece 16 so that they are adjustable towards or away from the armature 19 by means of a screwdriver operating-on the ends 31, 32, suitable locking means 33 being provided if required. Other methods, however, may be adopted and will readily suggest themselves.

The armature 19 preferably has an extension 34 at right angles thereto which is attached to a diaphragm 35 of any suitable form, and which in the example shown is of the type disclosed in British Patent No.

* 264,263. The armature, however, may be attached direct or otherwise to any kind of diaphragm, and when utilized in this way it may be maintained in its normal position by the diaphragm 35, which in the example is lightly mounted between gaskets 36, 37 so as to be under no strain when at rest.

In the modification shown in Figures 3 and 4 the electro-magnet is adapted to operate in a similar manner to the above example, but instead of forming one pole as a pot 1 construct the magnet so that the free end of the core 38 is of one polarity while the other end of the magnet is in the form of two limbs 39 and 40, which are of opposite polarity, the magnet being polarized by the field coil 41 as in the previous example.

The armature 19 may be formed from one piece of iron or the like or it may be built up of laminations in any suitable manner. It is pivoted upon the outer pole pieces 39 and 40 between the extensions 42 and 43, and in the example shown the pivot 44 may be slidable in the pole piece 43 and locked in place by the screw 45 when correctly adjusted relative to the ball or other suitable bearing 46. V

The central core is provided with two pole extensions 25 and 26 which are adjustable to or from the armature 19 by means of the screwed ends 31 and 32. Two operating coils 29 and 30 are located upon these pole extensions and are electrically connected together to operate either in parallel or in series as shown.

The armature 19 is connected through the lever 34 to the diaphragm 35, it being preferable that the pivot 44 (both in this and in the preceding modification) lies in approximately the same plane as the diaphragm 35 so that there shall be as little lateral displacement of the diaphragm as possible when vibrated under the influence of the armature. A certain amount of recovery or damping is exerted by the diaphragm 35 on the arma ture 19, and when required additional damping or recovery may be given to the armature by means of springs or the like, one form of which may be obtained by using a rubber pad 47 which is adjustable by the screw 48. Such an arrangement might conveniently be mounted on each side of the pivot 44 to adjust the armature 19 in either direction.

The spool 49 on which. the field coil 41 is wound may, if required, be formed in brass or any non-magnetic metal to minimize eddy current effects on the operating coils 29 and 30, or as in the example shown in Figures 1 and 2, a metal plate 50 may be provided in addition to the spool 49 for the same purpose.

Figures 5 and 6 show another construction of loud-speaker in which a number of modifications of the invention are indicated.

The core 51 of the electro-magnet carries the field coil 52 and is mounted upon a pillar 53 of magnetic material in such a manner that the core 51 together with the pole extensions 54 and 55 can be adjusted relative to the armature 56 by rotation of the adjusting screw 57 the spring 58-taking up any slackness between the parts 51 and 53. .The pillar 53 is formed as part of the magnetic system which extends through the base plate 59, the pillar 60 and thetop plate 61, and terminates in twoextensions 62 and 63 upon which the armature 56 is pivoted in such a manner that it is polarized by the magnet system to the opposite polarity of the extensions 54 and 55 of the same electro-magnetic system.

The armature 56 may be pivoted upon the pole ends 62 and 63 by means of balls or in any other desired manner, or as shown in Figures 5 and 6 the armature may be pivoted by means of a leaf spring 6 1, which also provides a certain amount of spring recovery to the armature to which it is a tached by screw 65 or by other suitable means. The spring is attached to the pole pieces 62 and 63 by screws 66 or the like, a very small gap being left between the pole pieces 62 or 63 and armature 56 so as to ensure proper polarization of the armature. The ends 62 and 63 are also cut away slightly at 67 to permit thespring 64: to flex sufficiently to enable the armature to pivot or rock relative to the pole pieces 62 and 63.

A cone-shaped diaphragm 68 of paper or other suitable material is attached to the armature at 69, the outer edge of the diaphragm being attached in any known manner to the rim 70, which is carried by the case 71 attached to the magnet system 56 and 61 by means of screws or the like.

Operating coils 72 and 73 are mounted upon the pole extensions 54: and 55 and oper-' ate in the manner already described with reference to the previous examples.

In the example shown in Figure 7 the armature 7 1 is attached to the central pole piece 7 5 by means of any suitable pivot 7 6 in such a manner that it is thereby polarized, and can vibrate freely to or from the adjustable pole extensions 77 and 78 which are mounted upon the outer limbs 79 and 80 of the elect-ro-magnet. The field coil 81 is mounted upon the central or core pole piece 7 5 and the operating coils 82 and 83 are mounted upon the adjustable pole extensions 77 and 78 as in the previous examles. 1 The armature is attached to a suitable diaphragm 84; through the connection 85, and the device operates in a similar manner to that described with reference to Figures 1 and 2, but the arrangement is not considered so efficient as those previously described, and is therefore merely shown by way of example of an alternative construction, as is also Figure 8 which shows an adaptation suitable for operating a recording stylus 86 in place of a diaphragmwhich could be attached if required.

The field magnet 87 in Figure 8 is mounted upon a magnet system 88, 89 and 90 in such a manner that it polarizes the armature 91 to a polarity opposite to that of the pole extensions 92, which are adjustable to or from the armature by adjustment of the screw 93.

The operating coils 94 and 95 are here shown mounted one on each side of the pivot 96 on the armature 91 instead of on the pole extensions 92 as in the previous examples, the armature 91 being maintained in its balanced position relative to the pole extensions 92 by means of the rubber pad 97 mounted on the armature extension 98 and adjustable by the screws 99.

All of the above examples are adapted to be operated in a similar manner, and the electrical impulses may obviously be derived from any suitable source, and may be amplified by a thermionic amplifier. They may for instance be received from a wireless re- 9 ceiving apparatus or they may be taken from an electrical reproducer or from an amplifier.

Additional resistances either in parallel or in series with the field coil or even the field coil itself can constitute a choke for by- Figures 9 to 12 show a number of circuit arrangements particularly adaptable to the devices described aboveand to sound-repro ducing apparatus having a field coil and one or more audio frequency operating coils.

In Figure 9, 100 represents the last stage of i l a valve amplifier, and current from the 101 is passed through the field coil F wound around the magnet M. When operated in this way the field coil F functions as choke, and if a condenser 102 is inserted in the circuit, the audio frequency current component from the anode 101 can be passed through the operating coils O and O and thereby operate the armature or diaphragm, while the direct current component passes through the field coil F to polarize same as described.

In Figure 10 the current from the valve V first passes through the audio frequency transformer 103 by which the audio frequency impulses are fed to the operating coils O and 3 O which are shown in parallel instead of in series as in the previous example. The direct current component is then passed through the field coil F to polarize the magnet M. while the condenser 10 1, also by-passes certain audio frequency impulses to obtain tone modulation or other desired effects. By adjustment of the tapped shunt resistance 105 it is possible to vary the volume of the reproduction, by varying the amount of current which is passed through the field coil F and thereby varying the strength of the magnet M.

Figure 11 shows a circuit in which better control is obtained over the relative values of the reproduced tone, the current from the valve V first passing through the choke 106 whereby the audio frequency current component is pass-ed through the condenser 107 and operating coil 0, while the direct current component is passed through the field coil F, the condenser 10% functioning as in Figure 10.

In Figure 12 an arrangement is shown whereby two sets of valves V and V can be used in push-pull fashion. The direct current component from the valves passes through the field coils F anc I, while the audio frequency current component passes through the condensers G and G and thereby through the operating coils Q and 0'.

On all of the above arrangements the volume of sound may be regulated by varying the field winding current, and this regulation may be obtained by any well-known method such as a variable resistance either in parallel or in series with one or more polarizing field coils. It is also obvious that any of the above divided current arrangements, whereby the direct current is passed through the field coil while the audio frequency current Component is passed through the operating coil or coils, may be used in conjunction with a moving coil armature instead of or together with a moving iron armature.

Any of the above arrangements may be conveniently mounted to operate in combination with one or more horns, such an arrangement being shown in Figure 13, in which a loudspeaker 108 of the type illustrated in Figures 1 an d 2 is for example shown attached to a horn 109. An additional horn or sound directing devicev 110 may also be attached to the diaphragn'i-holdingrim if required, or a sound directin device may be used instead of the long horn 109. It is obvious that the horn or sound directing devices may be shaped in many different ways, and that the diaphragm may be of any suitable size.

It is to be understood that the invention is not to be limited to the foregoing details of construction which are given by way of illustration only, and component parts may be multiplied, modified, or equivalent arrangements used to suit the particular purpose for which the device is to be employed or any practical requirements that may have to be fulfilled while at the same time the novel features of construction hereinbefore described may be reversely operated or combined with other and known components without departing from the scope of the invention.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is 1. In combination, an electro-magnet of the pot type, pole piece extensions on the central core of said magnet, inwardly directed extensions on theouter casing of the magnet, and an armature pivotally mounted on said inwardly directed extensions whereby it is located in close relation to the pole piece extensions on the central core of the magnet.

2. In an electromagnetic energy translator for the interconversion of mechanical and electrical audio frequencies, an electromagnet of the pot type, inwardly directed extensions on the outer casing of the magnet, an armature pivotally mounted on said inwardly directed extensions, pole pieces extending through the central core of said magnet, and means whereby said pole pieces may be adjusted through said core toward and from the armature.

3. In an electromagnetic energy translator for the interconversion of mechanical and electrical audio frequencies, an electromagnet of the pot type, inwardly directed extensions on the outer casing of the magnet, an ar mature pivotally mounted on said inwardly directed extensions and disposed over the central core of the magnet, pole pieces extending through the central core of the magnet, operating coils on said pole pieces and disposed between the armature and the core of the magnet, and means whereby said pole pieces may be adjusted through said core and operating coils toward and from the armature.

4. In combination, an electromagnet of the pot type, inwardly directed extensions on the outer casing of the magnet, an armature pivotally mounted on said inwardly directed extensions and disposed above the core of the magnet, pole pieces extending through the core of the magnet, operating coils on said pole pieces and disposed between the armature and the core of the magnet, and said pole pieces having screw threaded engagement with the core of the magnet where by they may be adjusted through said core and operating coils toward and from the armature.

5. In combination, an electromagnet in cluding a core and a pair of arms arranged at opposite sides of the core and extending above the same, an armature disposed above said core, means on said arms pivotally mounting the armature, pole pieces extending through said core and having their ends disposed in operative relation with the armature, and means whereby said pole pieces may be adjusted toward and from the armature.

6. In combination, an electromagnet including a core and'a pair of arms arranged at opposite sides of the core and extending to a point above the core, an armature disposed llil above the core and in line therewith, means on said arms pivotally mounting the armature, pole pieces extending through said core and having their ends disposed in operative relation with the armature, and said pole pieces having screw threaded engagement with said core whereby they may be adjusted toward and from the armature.

Signed at London, England, this 20th day of July, A. D. 1928.

FRANK ALLEN MITCHELL. 

